Battlefield Earth: You use everything from machine guns and sniper towers to mighty Sentinel powers to fight off the alien hordes in the tower defense game Sentinel 2. The tower defense genre has a large spectrum of flavors and a rich history on the mobile platform. The games are simple to pick up, addictive to play, and each has its own take on the genre. Sentinel 2 picks up after the destruction of Mars’ colonies by the invading alien hordes. Now you are tasked with the defense of Earth’s facilities. To aid you in this endeavor, you have a full range of defenses. Simple machine gun defenses, bomb throwers, slow towers, sniper towers, and many others can be upgraded and used to turn back the hordes of cookie-cutter aliens.

You earn cash by defeating the various waves of aliens or by hiring drones to gather resources for you. The resource-gathering aspect of the game is small, but integral for later stages. You can recruit drones to gather funds for you, repair your base, and eventually, help recharge your special Sentinel powers.

The Sentinel powers are arguably the most original aspect of the game, and are reminiscent of bonus powers or super weapons from real-time strategy games like Command and Conquer . When the hordes become too much, you can deploy one of four Sentinel powers that will aid you in combat. These include missile bombardments, an orbital canon, attack drones, and a glowing yellow eye that slows down your enemies in a wide area. They are recharged over time, but hiring drones to help with the process ensures they’ll be ready for redeployment faster.

In addition to standard assault mode, Sentinel 2 offers an endurance mode and special challenge missions. The endurance mode basically tasks you with surviving as long as possible against a never-ending series of attacking waves, but the challenge missions are more creative. Each mission tasks you with a special limitation on your abilities and thus enables for some creative strategizing. One mission requires you to take down a huge enemy unit with only limited resources while another has you taking out enemy forces using only Sentinel Powers.

Origin 8 has a partnership with OpenFeint , ensuring that you can share your top scores with your fellow players while introducing some social capabilities to what is primarily a single player game.

A typical assault mission will take you anywhere from 30 to 45 minutes to complete, with the later missions taking even longer. If you quit, your progress will be saved so you can pick it up later, but Sentinel 2 doesn’t offer a save system in game. The ability to pick up where you left off is key to this game, because most players will only have time for shorter play sessions. Sitting through an entire assault mission in one go-round can get tedious, and so speeding up the game’s pace (via a fast-forward button) is essential to not only fight off the alien hordes, but boredom.

Sentinel 2 offers a lot of firepower to deal with its thin selection of alien forces. Some of the game’s best moments are when your defenses are being overwhelmed, the aliens are knocking down your barriers, and you’re forced to call in a missile strike to wipe out the map. The weapons and the aliens aren’t particularly original, and neither are the powers, but combined the elements create an enjoyable, if at times tedious strategy experience.

Sentinel 2: Earth Defense Support is compatible with any iPhone or iPod touch running the iPhone 2.2 software update.

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